FTIR analysis of storage behavior and sulfur tolerance in barium-based NOx storage and reduction (NSR) catalysts

A series of NOx storage and reduction (NSR) catalysts containing platinum, BaO, and iron supported on gamma-Al2O3 has been studied using FTIR spectroscopy. CO adsorption studies at room temperature have shown that the BaO phase can chemisorb large quantities of CO, and that platinum sites can become encapsulated by BaO, following a reducing pretreatment. Exposure of the catalysts to fuel-lean and fuel-rich conditions has revealed that the species that participates in the storage is a surface nitrite complex (prevalent below 350degreesC), which is converted to a surface monodentate nitrate species above 350 degreesC. Both of these species have also been identified at room temperature and are bonded to the surface through BaO lattice oxygen atoms. A mechanism has been proposed to describe the storage and reduction process in which NO reacts directly with the BaO surface to form nitrite and monodentate nitrate. The role of platinum is to adsorb O-2 under lean conditions and reducing gases (H-2 or hydrocarbons) under rich conditions. Once adsorbed, the oxygen and reducing agents spill over onto nearby BaO sites and react with the adsorbed NO. The oxidation of NO to NO2 on platinum does not appear in this work to be a critical step, in contrast to previous studies. Sulfur tolerance studies using SO2 have confirmed the previous observation that promotion with iron increases the durability of NSR catalysts. However, FTIR results show that the improvement is caused by the formation of a previously unobserved bulk nitrate species and not to a decrease in the sulfate decomposition temperature. (C) 2003 Elsevier B.V. All rights reserved.